Chapter V. Brook and Atherfield

BROOK AND ATHERFIELD

To most Sandown Bay is by far the most accessible place in the Island to study the earlier strata; and for our first geological studies it has the advantage of showing a succession of strata so tilted that we can pass over one formation after another in the course of a short walk. But when we have learnt the nature of geological research, and how to read the record of the rocks, and examined the Wealden and Greensand strata in Sandown Bay, we shall do well, if possible, to make expeditions to Brook and Atherfield, to see the splendid succession of Wealden and Greensand strata shown in the cliffs of the south-west of the Island. It is a lonely stretch of coast, wild and storm-swept in winter. But this part of the Island is full of interest and charm to the lover of Nature and of the old-world villages and the old churches and manor houses which fit so well into their natural surroundings. The villages in general lie back under the shelter of the downs some distance from the shore; a coastguard station, a lonely farm house, or some fishermen's houses as at Brook, forming the only habitations of man we come to along many miles of shore. Brook Point is a spot of great interest to the geologist. Here we come upon Wealden strata somewhat older than any in Sandown Bay. The shore at the Point at low tide is seen to be strewn with the trunks of fossil trees. They are of good size, some 20 ft. in length, and from one to three feet in diameter. They are known as the Pine Raft, and evidently form a mass of timber floated down an ancient river, and stranded near the mouth, just as happens with great accumulations of timber which float down the Mississippi at the present day. The greater part of the wood has been replaced by stone, the bark remaining as a carbonaceous substance like coal, which, however, is quickly destroyed when exposed to the action of the waves. The fossil trees are mostly covered with seaweed. On the trunks may sometimes be found black shining scales of a fossil fish, Lepidotus Mantelli. (A stratum full of the scales of Lepidotus has been recently exposed in the Wealden of Sandown Bay.) The strata with the Pine Raft form the lowest visible part of the anticline. From Brook Point the Wealden strata dip in each direction, east and west. As the coast does not cut nearly so straight across the strata as in Sandown Bay, we see a much longer section of the beds. On either side of the Point are coloured marls, followed by blue shales, as at Sandown. To the westward, however, after the shales we suddenly come to variegated marls again, followed by a second set of shales. There was long a question whether this repetition is due to a fault, or whether local conditions have caused a variation in the type of the beds. The conclusion of the Geological Survey Memoir, 1889, rather favoured the latter view, on the ground of the great change which has taken place in the character of the beds in so short a distance, assuming them to be the same strata repeated. The conjecture of the existence of a fault has, however, been confirmed; for during the last years a most interesting section has been visible at the junction of the shales and marls, where a fault was suspected. The shales in the cliff and on the shore are contorted into the form of a Z. The section appears to have become visible about 1904 (it was in the spring of that year that I first saw it), and was described by Mr. R. W. Hooley, F.G.S. (Proc. Geol. Ass., vol. xix., 1906, pp. 264, 265). It has remained visible since.

The Wealden of Brook and the neighbouring coast is celebrated for the number of bones of great reptiles found here, from the early days of geological research, the '20's and '30's of last century, when admirable early geologists, such as Dr. Buckland and Dr. Mantell, were discovering the wonders of that ancient world, to the present time. Various reptiles have been found besides the Iguanodon--the Megalosaurus, a great reptile somewhat similar, but of lighter build, with sabre-shaped teeth, with serrated edges: the Hylaeosaurus, a smaller creature with an armour of plates on the back, and a row of angular spines along the middle of the back; the huge Hoplosaurus hulkei, probably 70 or 80 feet in length; the marine Plesiosaurus and Ichthyosaurus, and several more; also bones of a freshwater turtle and four types of crocodiles. In various beds a large freshwater shell, Unio valdensis, occurs, and in the cliffs of Brook have been found many cones of Cycadean plants. In bands of white sandy clay are fragments of ferns, Lonchopteris Mantelli. In the shales are bands of limestone with Cyrena, Paludina, and small oysters, and paper shales with cyprids, as at Sandown. The shore near Atherfield Point is covered with fallen blocks of the limestones.

The Lower Greensand is seen in Compton Bay on the northern side of the Brook anticline. Here is a great slip of Atherfield clay. The beds above the clay are much thinner than at Atherfield, and fossils are comparatively scarce. On the south of the anticline the Perna bed slopes down to the sea about 150 yards east of Atherfield Point, and runs out to sea as a reef. Large blocks lie on the shore, where numerous fossils may be found on the weathered surfaces. The ledges which here run out to sea form a dangerous reef, on which many vessels have struck. There is now a bell buoy on the reef. On the headland is a coastguard station, and till lately there has been a sloping wooden way from the top of the cliff to bring the lifeboat down. This was washed away in the storms of the winter 1912-13.

Above the Perna bed lies a great thickness of Atherfield clay. Above this lies what is called the Lower Lobster bed, a brown clay and sand, in which are numerous nodules containing the small lobster Meyeria vectensis,--known as Atherfield lobsters. Many beautiful specimens have been obtained.

We next come to a great thickness of the Ferruginous Sands, some 500 feet. The Lower Greensand of Atherfield was exhaustively studied in the earlier days of geology by Dr. Fitton, in the years 1824-47, and the different strata are still referred to according to his divisions. The lowest bed is the Crackers group about 60 ft. thick. In the lower part are two layers of hard calcareous boulder-shaped concretions, some a few feet long. The lower abound in fossils, and though hard when falling from the cliffs are broken up by winter frosts, showing the fossils they contain beautifully preserved in the softer sandy cores of the concretions. Gervillia sublanceolata is very frequent, also Thetironia minor, the Ammonite Hoplites deshayesi, and many more. Beneath and between the nodular masses caverns are formed, the resounding of the waves in which has given the name of the "Crackers." In the upper part of this group is a second lobster bed.

The most remarkable fossils in the Lower Greensand are the various genera and species of the ammonites and their kindred. The Ammonite, through many formations, was one of the largest, and often most beautiful shells. There were also quite small species. The number of species was very great. Now the whole group is extinct. They most resembled the Pearly Nautilus, which still lives. In both the shell is spiral, and consists of several chambers, the animal living in the outer chamber, the rest being air-chambers enabling it to float. The class Cephalopoda, which includes the Ammonites, the Nautilus, and also the Cuttle-fish, is the highest division of the Mollusca. The animals all possess heads with eyes, and tentacles around the mouth. They nearly all possess a shell, either external, as in the Nautilus, or internal, as in the cuttle-fishes, the internal shell of which is often washed ashore after a rough sea. The Cephalopods are divided into two orders. The first includes the Cuttle-fish and the Argonaut or Paper Nautilus. Their tentacles are armed with suckers, and they have highly-developed eyes. They secrete an inky fluid, which forms sepia. The internal shell of extinct species of cuttle-fish, of a cylindrical shape, with a pointed end, is a common fossil in various strata, and is known as a Belemnite (Gr. $belemnon$ "a dart".) The second order includes the Pearly Nautilus of the present day, and the numerous extinct Nautiloids and Ammonoids. The tentacles of the Pearly Nautilus have no suckers; and the eyes are of a curiously primitive structure,--what may be called a pin-hole camera, with no lens. The shells of the Nautilus and its allies are of simpler form, while the Ammonites are characterised by the complicated margins of the partition walls or septa, by which the shells are sub-divided. The chambers of the fossil Ammonites have often been filled with crystals of rich colours; and a polished section showing the chambers is then a most beautiful object.

Continuing along the shore, we come to the Lower Exogyra group, where Terebratula sella is found in great abundance. A reef with Exogyra sinuata runs out about 350 yards west of Whale Chine. The group is 33 ft. thick, and is followed by the Scaphites group, 50 ft. The beds contain Exogyra sinuata, and a reef with clusters of Serpulae runs out from the cliff. In the middle of the group are bands of nodules containing Macroscaphites gigas. The Lower Crioceras bed (16 ft.) follows, and crosses the bottom of Whale Chine. The Scaphites and Crioceras are Cephalopoda, related to the Ammonites; but in this Lower Cretaceous period a remarkable development took place; many of the shells began to take curious forms, to unwind as it were. Crioceras, a very beautiful shell, has the form of an Ammonite, but the whorls are not in contact; thus making an open spiral like a ram's horn, whence its name (Gk. $keras$, ram, $krios$, horn). Ancyloceras begins like Crioceras, but from the last whorl continues for some length in a straight course, then bends back again; Macroscaphites is similar, but the whorls of the spiral part are in contact. In Scaphites, a much smaller shell, the uncoiled part is much shorter, and its outline more rounded. It is named from its resemblance to a boat (Gk. $skaphe$).

The Walpen and Ladder Clays and Sands (about 60 ft.) contain nodules with Exogyra and the Ammonite Douvilleiceras martini. The dark-green clays of the lower part form an undercliff, on to which Ladder Chine opens. The Upper Crioceras Group (46 ft.), like the Lower, contains bands of Crioceras? also Douvilleiceras martini, Gervillia, Trigonia, etc. It must be stated that there is some uncertainty with regard to the ammonoids found in this neighbourhood, Macroscaphites having been described as Ancyloceras, and also sometimes as Crioceras. The discovery of the true Ancyloceras (Ancyloceras Matheronianum) at Atherfield is described (and a figure given) by Dr. Mantell; but what is the characteristic ammonoid of the "Crioceras" beds requires further investigation. The neighbourhood of Whale and Walpen Chines is of great interest. Ammonites may be found in the bottom of Whale Chine fallen out of the rock. Red ferruginous nodules with Ammonites lie on the shore, in the Chines, and on the Undercliff, some of the ammonites more or less converted into crystalline spar. Hard ledges of the Crioceras beds run into the sea. The shore is usually covered deep with sand and small shingle; but there are times when the sea has washed the ledges clear; and it is then that the shore should be examined.

The Walpen and Ladder Sands (42 ft.); the Upper Exogyra Group (16 ft.); the Cliff End Sand (28 ft.); and the Foliated Clay and Sand (25 ft.), consisting of thin alternations of greenish sand and dark-blue clay, follow. Then the Sands of Walpen Undercliff (about 100 ft.); over which lie the Ferruginous Bands of Blackgang Chine (20 ft.). Over these hard beds the cascade of the Chine falls. Cycads and other vegetable remains are found in this neighbourhood. Throughout the Atherfield Greensand fragments of the fern Lonchopteris (Weichselia) Mantelli are found. 220 ft. of dark clays and soft white or yellow sandrock complete the Lower Greensand. In the upper beds of the Greensand few organic remains occur. A beautiful section of Sandrock with the junction of the Carstone is to be seen inland at Rock above Bright-stone. The Sandrock here is brightly coloured like the sands of Alum Bay,--though it belongs to a much older formation,--and shows current bedding very beautifully. The junction of the Sandrock and Carstone is also well seen in the sandpit at Marvel.

We have now come to the end of the Lower Cretaceous, in which are included the Wealden and the Lower Greensand. Judged by the character of the flora and fauna, the two form one period, the main difference being the effect of the recession of the shore line, due to the subsidence which let in the sea over the Wealden delta, so that we have marine strata in place of freshwater deposits. But that the plants and animals of the Wealden age still lived in the not distant continent is shown by the remains borne down from the land. These strata are an example of a phenomenon often met with in geology,--that of a great thickness of deposits all laid down in shallow water. The Wealden of the Isle of Wight are some 700 feet thick, in Kent a good deal thicker, the Hastings Sands, the lower part of the formation, being below the horizon occurring in the Island: the Lower Greensand is some 800 feet thick. In the ancient rocks of Wales, the Cambrian and Silurian strata, are thousands of feet of deposits, mostly laid down in fairly shallow water. In such cases there has been a long-continued deposition of sediment, while a subsidence of the area in which it was laid down has almost exactly kept pace with the deposit. It is difficult not to conclude that the subsidence has been caused by the weight of the accumulating deposit,--continuing until some world-movement of the contracting globe has produced a compensating elevation of the area.